Ferromagnetic semiconductor materials and spintronic transistors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Hitachi Cambridge, Univ. Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Byonguk Park, Sam Owen, et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al. University of Texas Allan MaDonald, et al. Texas A&M Jairo Sinova, et al.

Electric field controlled spintronics HDD, MRAM controlled by Magnetic field Spintronic Transistor control by electric gates Low-voltage controlled magnetization and magnetotransport STT MRAM spin-polarized charge current From storage to logic Magnetic race track memory

Outline 1) Sensitivity to electric fields via magnetic anisotropies 1) Sensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit coupling generic to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device - Tunneling AMR device - Coulomb blockade AMR spintronic SET - Coulomb blockade AMR spintronic SET 2) Direct charge depletion effects on electric&magnetic proprties 2) Direct charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET

AMR TMR TAMR FM exchange int.: Spin-orbit int.: FM exchange int.: Au Discovered in GaMnAs Gould et al. PRL’04

ab intio theory TAMR is generic to SO-coupled systems including room-T c FMs experiment Bias-dependent magnitude and sign of TAMR Shick et al PRB ’06, Parkin et al PRL ‘07, Park et al PRL '08 Park et al PRL '08

& electric & magnetic control of Coulomb blockade oscillations SourceDrain Gate VGVG VDVD Q Devices utilizing M-dependent electro-chemical potentials: FM SET SO-coupling   (M) [ 010 ] M [ 110 ] [ 100 ] [ 110 ] [ 010 ]

(Ga,Mn)As nano-constriction SET CB oscillations shifted by changing M (CBAMR) Electric-gate controlled magnitude and sign of magnetoresistance  spintronic transistor or Magnetization controlled transistor characteristic (p or n-type)  programmable logic Wunderlich et al, PRL '06

Outline 1) Sensitivity to electric fields via magnetic anisotropies 1) Sensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit coupling generic to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device - Tunneling AMR device - Coulomb blockade AMR spintronic SET - Coulomb blockade AMR spintronic SET 2) Direct charge depletion effects on electric&magnetic proprties 2) Direct charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET

Mn-d-like local moments As-p-like holes Mn Ga As Mn EFEF DOS Energy spin  spin  Ferromagnetic semiconductor GaAs:Mn FM due to p-d hybridization (Zener kinetic-exchange) valence band As-p-like holes As-p-like holes localized on Mn acceptors << 1% Mn ~1% Mn >2% Mn onset of ferromagnetism near MIT (Ga,Mn)As: - heavily-doped SC  difficult to grow and gate - dilute moment FM  difficult to achieve high T c Jungwirth et al, RMP '06

(Ga,Mn)As growth Low-T MBE to avoid precipitation & high enough T to maintain 2D growth  need to optimize T & stoichiometry for each Mn-doping high-T growth optimal-T growth Annealing also needs to be optimized for each Mn-doping Detrimental interstitial AF-coupled Mn-donors  need to anneal out (T c can increase by more than 100K)

T c up to 187 K at 12% Mn doping No indication for reaching technological or physical T c limit in (Ga,Mn)As yet Novak et al. PRL ‘ Growth & post-growth optimized GaMnAs films

Weak hybrid. Delocalized holes long-range coupl. Strong hybrid. Impurity-band holes short-range coupl. InSb GaP d5d5 GaAs seems close to the optimal III-V host Other (III,Mn)V’s DMSs Mean-field but low T c MF Large T c MF but low stiffness Kudrnovsky et al. PRB 07

Magnetism in systems with coupled dilute moments and delocalized band electrons coupling strength / Fermi energy band-electron density / local-moment density Jungwirth et al, RMP '06

III = I + II  Ga = Li + Zn Other DMS candidates Masek et al. PRL 07 But Mn isovalent in Li(Zn,Mn)As  no Mn concentration limit and self-compensation  possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry) GaAs and LiZnAs are twin SC (Ga,Mn)As and Li(Zn,Mn)As should be twin ferromagnetic SC

Towards spintronics in (Ga,Mn)As: FM & transport Ordered magnetic semiconductors Eu  - chalcogenides Disordered DMSs Sharp critical contribution to resistivity at T c ~ magnetic susceptibility Broad peak near T c and disappeares with annealing (higher uniformity)??? 

TcTc TcTc Eu 0.95 Cd 0.05 S Ni, Fe

Sharp d  /dT singularity in GaMnAs at T c – consistent with F ~d  -  Novak, et al. PRL‘08

Optimized GaMnAs materials with x~4- 12% and Tc~80-185K: well behaved FMs Annealing sequence t=(Tc-T)/Tc

As-p-like holes Strong spin-orbit coupling  favorable for spintronics Strong SO due to the As p-shell (L=1) character of the top of the valence band VV B eff p s Mn Ga As Mn

Low-voltage gating of the highly doped (Ga,Mn)As p-n junction depletion simulations ~25-50% depletion feasible at low voltages 2x cm -3 Owen, et al. arXiv: ’s-100’s Volts in conventional MOS FETs Ohno et al. Nature ’00, APL ‘06 p-n junction FET

Basic charcteristics of the device can deplete charge at low Vg can “deplete” magnetization at low Vg low Vg dependent competition of uniaxial and cubic anisotropies 30% AMR tuneable by low Vg

Magnetization switching by 10ms low-Vg pulses

Conclusion 1) Studies in GaMnAs suggest new generic approaches to 1) Studies in GaMnAs suggest new generic approaches to electric field controlled spintronics via magnetic anisotropies electric field controlled spintronics via magnetic anisotropies - TAMR - TAMR - CBAMR - CBAMR 2) Direct charge depletion effects on electric&magnetic properties 2) Direct charge depletion effects on electric&magnetic properties of GaMnAs demonstrated at low gate voltages of GaMnAs demonstrated at low gate voltages - GaMnAs junction FET - GaMnAs junction FET Mn Ga As Mn